SCID mouse models of human stem cell engraftment.

The discovery of the severe combined immunodeficiency (scid) mouse mutation has provided a tool for establishment of small animal models as hosts for the in vivo analysis of normal and malignant human pluripotent hemopoietic stem cells. Intravenous injection of irradiated scid mice with human bone marrow, cord blood, or G-CSF cytokine-mobilized peripheral blood mononuclear cells, all rich in human hemopoietic stem cell activity, results in the engraftment of a human hemopoietic system in the murine recipient. This model has been used to identify a pluripotent stem cell, termed "scid-repopulating cell" (SRC) that is more primitive than any of the hemopoietic stem cell populations identified using the currently available in vitro methodology. In this review, we describe the development and use of this model system, termed Hu-SRC-SCID, and summarize the discoveries that have resulted from the investigation of human stem cells in this model. Finally, we detail the recent extension of the original Hu-SRC-SCID model system based on the C.B-17-scid mouse as the murine host to the Hu-SRC-NOD-SCID model based on the NOD-scid mouse as the host. The engraftment of human stem cells in the Hu-SRC-NOD-SCID model is enhanced over that observed in the Hu-SRC-SCID model and results in exceptionally high levels of human hemopoietic cells in the murine recipient. Future directions to further improve the Hu-SRC-NOD-SCID model system and the potential utility of this model in the preclinical and diagnostic arenas of hematology and oncology are discussed.

Blockade of CD40-CD154 interferes with human T cell engraftment in scid mice.

Antibodies to the ligand for CD40 (CD154) have been shown to exert profound effects on the development of cell-mediated immune responses in mice. The present study shows that an antibody to human CD154 (hCD40L) inhibits in vivo Tetanus toxoid (TT) specific secondary antibody responses in hu-PBL-scid mice, as well as the expansion of xenoreactive human T cells in the scid mice. A possible cause for the reduced expansion of xenoreactive, human T cells, was the decreased expression of murine B7.1 and B7.2 caused by the administration of anti-hCD40L. Therefore, it may be that defective maturation of murine antigen-presenting cells impeded the priming and expansion of human xenoreactive T cells.

Enhanced human CD4+ T cell engraftment in beta2-microglobulin-deficient NOD-scid mice.

Genetic crosses produced NOD/LtSz mice doubly homozygous for the severe combined immunodeficiency (scid) mutation and the beta2m (B2m) null allele. Both NOD/LtSz-scid/scid and NOD/LtSz-scid/scid B2m(null) mice lacked mature lymphocytes and serum Ig. However, homozygosity for the B2m(null) allele also resulted in the absence of MHC class I expression, loss of NK cell activity, accumulation of iron in the liver, and rapid clearance of human IgG1. NOD/LtSz-scid/scid B2m(null) mice supported markedly elevated levels of human T cell engraftment, compared with NOD/LtSz-scid/scid control animals, following injection with human PBMC. The increased engraftment was associated with a major increase in the number of human CD4+ T cells. Following injection with 20 million human PBMC, levels of human CD4+ T cells in the peripheral blood and spleen of NOD/ LtSz-scid/scid B2m(null) mice were 6- to 7-fold higher than those in NOD/LtSz-scid/scid mice and >50-fold higher than those in C.B-17-scid/scid mice. The resulting normalization of CD4+/CD8+ ratios in NOD/LtSz-scid/scid B2m(null) mice is in sharp contrast to that observed in NOD/LtSz-scid/scid mice or in C.B-17-scid/scid mice. Circulating human IgG was cleared 6-fold more rapidly in NOD/LtSz-scid/scid B2m(null) mice than in NOD/LtSz-scid/scid mice. This rapid IgG clearance suggested a failure of the engrafted human lymphoid cells to maintain high circulating levels of human IgG. The higher levels of human CD4+ T cells and the normalization of the CD4:CD8 ratio that are observed in human PBMC-engrafted NOD/LtSz-scid/scid B2m(null) mice suggest that this system may be an excellent model for studies of HIV pathogenesis.